DIRAS3 regulates the autophagosome initiation complex in dormant ovarian cancer cells

DIRAS3 is an imprinted tumor suppressor gene that is downregulated in 60% of human ovarian cancers. Re-expression of DIRAS 3 at physiological levels inhibits proliferation, decreases motility, induces autophagy, and regulates tumor dormancy. Functional inhibition of autophagy with choroquine in dormant xenografts that express DIRAS 3 significantly delays tumor regrowth after DIRAS 3 levels are reduced, suggesting that autophagy sustains dormant ovarian cancer cells. This study documents a newly discovered role for DIRAS 3 in forming the autophagosome initiation complex (AIC) that contains BECN1, PIK3C3, PIK3R4, ATG14, and DIRAS3. Participation of BECN1 in the AIC is inhibited by binding of BECN1 homodimers to BCL2. DIRAS 3 binds BECN1, disrupting BECN1 homodimers and displacing BCL2. Binding of DIRAS 3 to BECN1 increases the association of BECN1 with PIK3C3 and ATG14, facilitating AIC activation. Amino acid starvation of cells induces DIRAS 3 expression, reduces BECN1-BCL2 interaction and promotes autophagy, whereas DIRAS 3 depletion blocks amino acid starvation-induced autophagy. In primary ovarian cancers, punctate expression of DIRAS 3, BECN1, and the autophagic biomarker MAP1LC3 are highly correlated (P < 0.0001), underlining the clinical relevance of these mechanistic studies. Punctate expression of DIRAS 3 and MAP1LC3 was detected in only 21-23% of primary ovarian cancers but in 81-84% of tumor nodules found on the peritoneal surface at second-look operations following primary chemotherapy. This reflects a 4-fold increase (P < 0.0001) in autophagy between primary disease and post-treatment recurrence. We suggest that DIRAS 3 not only regulates the AIC, but induces autophagy in dormant, nutrient-deprived ovarian cancer cells that remain after conventional chemotherapy, facilitating their survival.